This invention relates to a variable valve timing mechanism for use in controlling gaseous flow and pressure to or from fluid displacement apparatus used in various applications including fuel cell systems.
In order to provide a wide range of power output, fuel cell powering systems need to have precise and independent control of the mass flow and pressure of gases used in the fuel cell process. For automotive applications, where minimizing the size and weight of the powering system is also important, the use of high gaseous pressure to obtain adequate power for the vehicle is generally regarded as a superior approach. However as pressure needs increase, the generation of pressure is more problem filled. In order to minimize the energy cost of generating high inlet pressures using a compressor, there is general agreement that an expander should also be used to recover unused energy and thereby improve system efficiency.
While recovery of energy is important for system efficiency, it is not possible to have a well matched compressor/expander pair, except at one operating point, unless there is an ability to modify the displaced volume of the compressor, the expander or both, during operation. When the compressor and expander are not well matched, their combined efficiency drops rapidly resulting in a similar decrease in the efficiency of the fuel cell powering system. When a fuel cell powering system is used to power vehicles, it must meet the dynamic power requirements of the vehicle, it must contribute to good fuel economy, and it must aid in meeting the emissions requirements typical of automotive applications. To accomplish such objectives, the fuel cell system must provide an energy efficient level of performance over a wide operating range. Since the gaseous fluid supply system represents one of the largest parasitic energy costs of the fuel cell powering system, it is especially important to provide a well matched, compressor/expander pair over the same wide operating range.
The ability of fluid delivery and energy recovery apparatus (hereafter fluid supply apparatus), compressors and expanders, to pump fluids (volumetric efficiency) is highly dependent upon the speed of the apparatus as well as the pressure requirements of the using devices. Typically, as either speed is decreased or pressure increased, the volumetric efficiency of the fluid supply apparatus deteriorates. Such a result causes an increase in energy consumption in the compressor and/or a decrease in energy recovery from the expander. An ideal compressor/expander pairing for a vehicular fuel cell application would show little change in volumetric efficiency as speed or system pressure requirements change.
Some types of fluid supply apparatus are better suited to the delivery of fluid over a wide range of pressure than other types. Low leakage fluid supply apparatus such as sealed pistons, sealed diaphragms, and sealed scrolls have a significantly greater pressure range than clearance roots, turbo-compressors, clearance twin screws and similar fluid supply apparatus. However, all compressors and expanders, even those with seals, exhibit the characteristic of changing volumetric efficiency, as speed and pressure requirements change. These unavoidable changes in the ability of the apparatus to move a specific amount of fluid at a specific pressure at a specific speed complicates the establishment of an adequate fuel cell drive system for applications such as automotive use. The typical non-linear behavior of compressors and expanders further increase the difficulty when both are present in the powering system and should be matched for system efficiency as noted above. Therefore, what is needed is an independent, variable control mechanism to compensate for volumetric efficiency changes as well as to provide for a user selectable system pressure capability at all operating conditions. One solution is the variable displacement piston fluid supply apparatus described in U.S. Pat. Nos. 4,907,950, 5,004,404 and 5,114,321, incorporated herein by reference. That apparatus has demonstrated the required pressure control capability and compensation for changes in volumetric efficiency. This invention is for an alternative or complementary mechanism to accomplish the objectives through variable valve timing to achieve dynamic modulation of the displaced volume of a compressor and/or expander.
Briefly stated, this invention is a mechanism, system and method for controlling mass flow and pressure for systems utilizing fluid supply apparatus, and is intended particularly for meeting the stringent requirements of fuel cell powering systems used in vehicular applications. Control is achieved through a variable valve timing approach, and alternative embodiments are described.
One embodiment is a rotating disc valve positioned above the inlet and discharge ports of the fluid supply apparatus. The disc contains slots that are aligned with the ports to connect or block connection of the ports to associated delivery or exhaust lines, depending on the position of the slots in the rotating disc. Multiple slots are used to reduce the speed of disc rotation. A movable plate(s) is used to provide an adjustable timing edge to modify the opening and/or the closing of the ports. In that manner, the positioning of the timing edge is independent of the rotational speed of the rotating disc and is also independent of the rotational speed of the compressor and/or expander. If multiple compressors and/or expanders are included in a system, each compressor and expander can be independently controlled since each can be provided with an independently controllable timing edge. The positioning of the timing edge is dynamically controllable by making it a part of a servo system.
A second embodiment is a rotary cylinder valve with a movable timing edge. Other embodiments utilize reciprocating valve motion instead of rotary motion and do not require a timing edge. In these embodiments an actuator capable of stepping motion in both directions is utilized to move the valve structure to open and close position for periods of time that are variable in accordance with actuator control.